An aluminum alloy is lightweight and has high heat conductivity, and thus it is used in a heat exchanger for an automobile, for example, a radiator, a condenser, an evaporator, a heater core, or an intercooler.
In such a heat exchanger, for example, it has been, heretofore, utilized a fin of an aluminum alloy that has been formed in a corrugated form by corrugation forming, in a state of being brazed (braze-joined). Regarding the aluminum alloy fin material, use has been usually made of: pure aluminum-based alloys excellent in thermal conductivity, such as JIS 1050 alloys; and Al—Mn-based alloys excellent in mechanical strength and buckling resistance, such as JIS 3003 alloys.
In recent years, there is an increasing demand for weight reduction, size reduction, and performance enhancement, for heat exchangers. Along with this demand, it is particularly desired for aluminum alloy fin materials that are brazed, to have a small thickness and to have excellent characteristics, such as mechanical strength after braze-heating, thermal conductivity, and corrosion resistance.
However, as making the fin material thinner (sheet metal gauging of the fin material) proceeds, enhancement in mechanical strength is also demanded. Along with that demand, there occurs a problem that the mechanical strength before braze-heating enhances, and it is difficult to have a predetermined dimension when the fin material is worked into a fin by corrugation forming.
Patent Literature 1 proposes a high-mechanical strength aluminum alloy fin material having a sheet thickness of 40 to 200 μm, which is cast by a twin belt-type continuous casting and rolling method, and which has a fibrous microstructure before braze-heating. However, since recrystallization is not carried out upon intermediate annealing, and the metallographic microstructure before braze-heating is a fibrous microstructure, the strain amount of the fin material in the raw material state is made large. As a result, the raw material strength is made high, and when a fin material having a small thickness is subjected to corrugation working, a predetermined dimensional accuracy cannot be obtained, and there is a risk that the performance of the resultant heat exchanger may deteriorate.
Patent Literature 2 proposes a drooping resistant fin material having a sheet thickness of less than 0.2 mm, which is obtained by: casting the raw material by a twin roll-type continuous casting and rolling method; setting the final cold-rolling reduction ratio to 60% or more; and subjecting the fin material having the final sheet thickness to final annealing. However, in order to suppress drooping upon the braze-heating, final cold-rolling is carried out at a rolling reduction ratio of 60% or more, and the raw material strength before the braze-heating is further set by the final annealing. As a result of carrying out the annealing, flatness in the coil's transverse becomes conspicuously poor, and the product quality or productivity upon the final slitting step is deteriorated to a large extent.
Patent Literature 3 proposes a high mechanical strength aluminum alloy material for an automotive heat exchanger having a final sheet thickness of 0.1 mm or less and having excellent formability and erosion resistance, which is obtained by: casting by a continuous casting and rolling method, and in which the proportion of a fibrous microstructure in the microstructure before braze-heating is 90% or more or 10% or less, and in which the density of dispersed particles having a circle-equivalent diameter of 0.1 to 5 μm in the aluminum alloy material surface before braze-heating is defined. However, although the proportion of the fibrous microstructure in the microstructure before braze-heating is defined, if the fibrous microstructure remains as described above, the raw material strength is made high, and there is a risk that the corrugation formability may be deteriorated. Further, if a recrystallized microstructure has no residual fibrous microstructure, it is necessary to set the temperature of the intermediate annealing to a high temperature. Thus, second phase particles become coarse upon the annealing to have a sparse distribution, and the mechanical strength after braze-heating is lowered.
Patent Literature 4 proposes a method of producing a high strength aluminum alloy material for an automotive heat exchanger having a final sheet thickness of 0.1 mm or less and having excellent erosion resistance, the method containing: casting the alloy raw material by a continuous casting and rolling method; and carrying out the first annealing at a temperature of 450° C. to 600° C. for 1 to 10 hours. However, since the intermediate annealing is carried out at a high temperature, second phase particles become coarse upon the annealing to have a sparse distribution as described above, and the mechanical strength after braze-heating is lowered.
Patent Literature 5 proposes an aluminum alloy fin material for a heat exchanger having a final sheet thickness of 40 to 200 μm, which is obtained by: casting the fin raw material by a twin belt-type continuous casting method; and carrying out first intermediate annealing at a temperature of 250° C. to 550° C. and second intermediate annealing at a temperature of 360° C. to 550° C. However, no metallographic microstructure before braze-heating is defined, the raw material strength is made high, and thus, there is a possibility that the corrugation formability may be deteriorated.
Further, in Patent Literatures 1 and 5, a twin belt-type continuous casting and rolling method is employed as the casting method. However, a twin belt system is characterized in that the cooling speed at the time of casting is slower than a twin roll system due to the difference in the casting method. Thus, for example, when an alloy containing Fe is cast, since Fe has a very low solid solubility in aluminum, most of Fe is crystallized out at the time of casting to form Al—Fe-based second phase particles (for example, Al—Fe—Si-, Al—Fe—Mn-, and Al—Fe—Mn—Si-based compounds) in aluminum. Thus, when an alloy containing these elements is cast, the second phase particles are crystallized out in a coarse state, and there is a high possibility for accelerating abrasion of the die at the time of corrugation forming, which is industrially not preferable.